The present invention relates to a discharge lamp, a discharge lamp device using the discharge lamp, and a reader using the discharge lamp device.
Conventionally, an external electrode type discharge lamp that uses no mercury and is superior in the rise characteristics of a beam of light is known as a discharge lamp used in, for example, an image reader, such as a copying machine or an image scanner, as mentioned in, for example, Published U.S. Pat. No. 2,969,130.
In this discharge lamp, the interior of a tube-shaped light emitting body, such as a glass tube, is filled with a discharge medium, such as xenon, and the outer face of the light emitting tube is provided with a pair of external electrodes in such a way as to face each other, and, by applying a voltage between the pair of external electrodes and passing an electric current through them, the discharge medium discharges in the light emitting tube, thereby outwardly emitting luminous light generated by an electric discharge. A fluorescent layer is formed on the inner wall surface of the light emitting tube, except for the area of an aperture portion between the pair of external electrodes. The fluorescent materials of the fluorescent layer are excited by ultraviolet rays emitted by the electric discharge of the discharge medium, the ultraviolet rays are then converted into visible light, and the visible light is projected outwardly through the aperture portion.
However, in the discharge lamp that uses the pair of external electrodes, two tube walls on both sides of the light emitting tube are placed between the pair of external electrodes, and the tube walls serve to limit an electric current flowing between the external electrodes. Therefore, in order to obtain current by which an electric discharge is activated or lighting is maintained, a high voltage of about 2 to 3 kV and a high frequency of several tens of kilohertz to several hundred kilohertz, for example, are needed for lamp input. Thus, there is a problem in that a high-pressure proofing constituent element needs to be used as a lighting circuit part when the lamp input becomes a high voltage and, in addition, the electrodes to which the high voltage is applied must be coated to be fully insulated. Further, there is another problem in that a high frequency increases the emission of electromagnetic waves and thereby exerts a noise influence upon other electronic equipment although it might be a possible solution to raise a lighting frequency instead of considerably raising the lamp input voltage.
As mentioned in Japanese Unexamined Patent Publication No. 27269 of 1995, there is a discharge lamp in which a shaft-shaped internal electrode is disposed at the center of a cross section of a tube-shaped light emitting body, which has been sealed, along the longitudinal direction of the light emitting tube, and a high frequency voltage is applied between the internal electrode and an external electrode disposed on the outer face of the light emitting tube, thereby discharging between the internal and external electrodes. As the external electrode, use is made of a wire mesh or a metal film impervious to light that is disposed at an area excluding an aperture portion.
However, in the discharge lamp that uses the shaft-shaped internal electrode, only one tube wall of the light emitting tube is placed between the shaft-shaped internal electrode disposed at the center of the cross section of the light emitting tube and the external electrode, and therefore the starting voltage can be lowered more than in a case where a pair of external electrodes are used, but there is a need to dispose the internal electrode at the center of the light emitting tube that is to be sealed. Therefore, the processing accuracy of the sealing must be improved, and, if the processing accuracy is low, characteristic fluctuations will easily occur. Additionally, since there is a need to dispose the shaft-shaped internal electrode at the center of the cross section of the bulb, the path length of an electric discharge cannot be lengthened to xc2xd or more of the inner diameter of the tube. Therefore, there is a problem in that it is difficult to greatly increase efficiency, and it is difficult to form the internal electrode into a desired shape.
In the discharge lamp that uses the pair of external electrodes and in the discharge lamp that uses the shaft-shaped internal electrode and the metal film impervious to light serving as an external electrode, the fluorescent layer is formed in the area excluding the aperture portion so as to emit light from the aperture portion. Therefore, there is a problem in that the conversion efficiency of ultraviolet rays into visible light is lower than a case where a fluorescent layer is formed on the whole of the inner wall surface of a light emitting tube so as to emit light from the whole of the light emitting tube as in fluorescent lamps used generally, and, in addition, the luminous efficiency is so low that only about 65% of visible light generated in the light emitting tube is emitted out of the light emitting tube.
Accordingly, a possible solution for improving the luminous efficiency of the light emitting tube is to use a transparent conductive film as an external electrode. However, in a conventional discharge lamp whose starting voltage is high, the film thickness of the transparent conductive film needs to be thickened because the electric resistance and loss of the transparent conductive film must be reduced. This leads to a drop in the visible light transmittance of the transparent conductive film, so that the luminous efficiency cannot be expected to be fully improved. And, in the discharge lamp mentioned in Japanese Unexamined Patent Publication No. 27269 of 1995, the metal mesh is used as an external electrode. However, the metal mesh is at a disadvantage in that adhesion between the glass of the light emitting tube and the metal mesh is bad, and a slight electric discharge occurs in the outer face of the light emitting tube, and, in addition, dust adheres to the mesh.
The present invention was made in view of these respects, and it aims to provide a discharge lamp capable of being easily manufactured, capable of reducing lamp voltage, such as a starting voltage or a discharge maintaining voltage, and capable of improving the luminous efficiency, to provide a discharge lamp device using the discharge lamp, and to provide a reader using the discharge lamp device.
A discharge lamp of the present invention comprises a tube-shaped light emitting body, a discharge medium enclosed in the light emitting tube, an internal electrode formed on an inner wall surface of the light emitting tube along a longitudinal direction of the light emitting tube, and an external electrode disposed outside the light emitting tube along the longitudinal direction of the light emitting tube. Since the internal electrode is formed on the inner wall surface of the light emitting tube, and the external electrode is disposed outside the light emitting tube, only one tube wall of the light emitting tube lies between the internal electrode and the external electrode, and therefore the limitation of an electric current running between the internal electrode and the external electrode can be reduced, and a lamp voltage, such as a starting voltage or a discharge-maintaining voltage, can be lowered. Further, since the internal electrode is formed on the inner wall surface of the light emitting tube, it can be easily processed with high accuracy. Further, since the internal electrode is disposed on the inner wall surface of the light emitting tube, the internal electrode and the external electrode can be arranged to create a desired relationship of being close to or away from each other.
Presumably, an electric discharge first starts where the distance between the internal electrode and the external electrode is shortest, and an electric-discharge path length is gradually lengthened when a voltage is applied therebetween. This fact explains that the starting voltage can be reduced.
Further, the internal electrode and the external electrode are placed so that at least part of them overlaps with each other with a tube wall of the light emitting tube therebetween. Since at least a part of the internal electrode and the external electrode overlaps with each other with tube wall of the light emitting tube therebetween, the distance therebetween can be made shortest, and the lamp voltage, such as starting voltage or discharge-maintaining voltage, can be lowered.
Further, the light emitting tube has an aperture portion through which a beam of light generated by an electric discharge in the light emitting tube is emitted to the outside, and the internal electrode is formed at one side of the aperture portion, and the external electrode is formed at a position excluding the aperture portion. Since the internal electrode is formed at one side of the aperture portion, and the external electrode is formed at the position excluding the aperture portion, the distance between the internal electrode and the external electrode can be shortened, and the lamp voltage, such as starting voltage or discharge-maintaining voltage, can be lowered.
Further, the internal electrode and the external electrode have a relationship of facing to each other with a sectional center of the light emitting tube therebetween. Since the internal electrode and the external electrode have the relationship of facing to each other with a sectional center of the light emitting tube therebetween, a positive column that passes through the sectional center of the light emitting tube between the internal electrode and the external electrode can be generated, and the luminous efficiency can be improved.
Further, an auxiliary external electrode that is not electrically connected to the external electrode is disposed outside the light emitting tube and in the vicinity of the internal electrode. Since the auxiliary external electrode that is not electrically connected to the external electrode is disposed outside the light emitting tube and in the vicinity of the internal electrode, electric power is supplied among the internal electrode, the external electrode, and the auxiliary external electrode, for example, when actuated, and therefore an electric discharge can be easily generated between the internal electrode and the auxiliary external electrode, and the starting voltage can be lowered.
Further, the external electrode has a transparent conductive film. Since the lamp voltage of the discharge lamp is lowered, the transparent conductive film that is thin in film thickness can be used as an external electrode, and the luminous efficiency can be improved.
Further, the external electrode has a primary conductive portion that is impervious to light, the primary conductive portion being connected to the transparent conductive film, with at least part of the primary conductive portion overlapping with the internal electrode with the tube wall of the light emitting tube therebetween. Since the external electrode has the primary conductive portion that is impervious to light, the primary conductive portion being connected to the transparent conductive film, with at least part of the primary conductive portion overlapping with the internal electrode with the tube wall of the light emitting tube therebetween, an electric discharge can be easily generated between the internal electrode and the primary conductive portion when actuated, and the electric resistance and loss of the external electrode can be reduced by using the primary conductive portion and the transparent conductive film together as an external electrode.
Further, the relation of 0.6 less than Sxc2x7T is created, where S is an aperture ratio excluding the primary conductive portion of the outer wall surface of the light emitting tube, and T is transmissivity of the transparent conductive film. If 0.6 less than Sxc2x7T, luminous efficiency that exceeds that of the discharge lamp having the aperture portion can be obtained.
Further, a discharge lamp of the present invention comprises a light emitting tube, a discharge medium enclosed in the light emitting tube, and a pair of internal electrodes formed to face each other with a sectional center of the light emitting tube therebetween along a longitudinal direction of the light emitting tube. Since the pair of internal electrodes are formed to face each other with the sectional center of the light emitting tube therebetween along the longitudinal direction of the light emitting tube, the tube wall of the light emitting tube does not lie between the internal electrodes, and the limitation of an electric current running between the internal electrodes can be reduced. In addition, the lamp voltage, such as starting voltage or discharge-maintaining voltage, can be lowered, and the internal electrode can be easily processed with high accuracy. In addition, a positive column that passes through the sectional center of the light emitting tube can be generated between the pair of internal electrodes, and the luminous efficiency can be improved.
An edge of at least one of the internal electrodes is ruggedly formed. Since the edge of at least one of the internal electrodes is ruggedly formed, an electric discharge stably concentrates on the convex part of the rugged portion when the electric field strength of the convex part of the electrode rises, and flickering can be prevented.
Further, a dielectric layer is formed on the inner wall surface of the light emitting tube so as to cover the internal electrode. Since the dielectric layer is formed on the inner wall surface of the light emitting tube so as to cover the internal electrode, the internal electrode can be prevented from sputtering because of an electric discharge, and the lifetime of the lamp can be lengthened.
Further, the dielectric layer is made up of a plurality of layers different from each other in softening-point. Since the dielectric layer is made up of a plurality of layers different from each other in softening-point, an electrode material is prevented from diffusing into an outer dielectric layer by an inner dielectric layer whose softening point is higher than that of the outer dielectric layer by making the softening point of the inner dielectric layer directly covering the internal electrode higher than that of the outer dielectric layer covering the inner dielectric layer when the outer dielectric layer is melted and burned, for example. The outer dielectric layer can be formed as a layer that has fewer pinholes and has a uniform film, and a withstand voltage of the dielectric layer can be secured, and the lamp life can be improved.
Further, the dielectric layer is covered with an electron emitting layer. Since the dielectric layer is covered with the electron emitting layer, the emission of electrons into the interior of the light emitting tube is facilitated by the electron emitting layer, and an electric discharge under the condition of a low lamp voltage can be allowed even if a dielectric layer is formed to cover the internal electrode.
Further, a discharge lamp device of the present invention comprises a discharge lamp provided with an auxiliary external electrode, and a lighting device for supplying electric power among the internal electrode, the external electrode, and the auxiliary external electrode of the discharge lamp when actuated and supplying electric power between the internal electrode and the external electrode after being actuated. Since the discharge lamp is provided with the auxiliary external electrode, and, by the lighting device, electric power is supplied among the internal electrode, the external electrode, and the auxiliary external electrode of the discharge lamp when actuated, and electric power is supplied between the internal electrode and the external electrode after being actuated, the electric discharge is easily carried out between the internal electrode and the auxiliary external electrode when actuated, and the starting voltage can be lowered.
Further, a discharge lamp device of the present invention comprises a discharge lamp, and a lighting device for lighting the discharge lamp with the external electrode of the discharge lamp as a grounding potential. Since the discharge lamp is lighted by the lighting device with the external electrode of the discharge lamp as the grounding potential, a high-potential external electrode is not disposed outside the light emitting tube, and the insulation process of the external electrode can be facilitated, and the generation of noise can be reduced.
The lighting device applies a direct-current pulse voltage in which the internal electrode is set to be a cathode side. Since the direct-current pulse voltage in which the internal electrode is set to be a cathode side is applied by the lighting device, the influence of ion collision upon the internal electrode can be reduced, and the lifetime of the lamp can be lengthened.
Further, a reader of the present invention comprises a carriage, a discharge lamp device in which at least a discharge lamp is mounted in the carriage, and a light receiving means for receiving reflected light from a projecting surface from which a beam of light of the discharge lamp is projected. The above-mentioned discharge lamp is applicable as a discharge lamp having a long electric-discharge path length like the reader.